When light shines onto our retinas, a signal is sent to the brain, which is in turn transmitted to a small gland. When this gland realises that daylight is fading, it produces melatonin, the hormone that makes us feel sleepy. Put simply, natural light levels control how much melatonin our bodies produce. If we're exposed to bright lights at night, our bodies produce less melatonin, making it harder to fall asleep.